Method of and apparatus for in-situ measurement of degradation of automotive fluids and the like by micro-electron spin resonance (ESR) spectrometry

ABSTRACT

A method of and miniaturized apparatus adapted for in-situ measurement of degradation of automotive fluids and the like by micro-electron spin resonance (ESR) spectrometry, wherein the use of a modulated constant magnetic field in an RF resonating variable frequency microwave cavity resonator through which a fluid sample is passed, enables direct detection of molecular changes in such fluid sample resulting from fluid degradation during use.

PRIORITY CLAIM

The present application is a divisional application of U.S. patentapplication Ser. No. 11/590,522, filed on Oct. 31, 2006 now U.S. Pat.No. 7,589,529, entitled, “Method Of And Apparatus For In-SituMeasurement of Degradation of Automotive Fluids And The Like ByMicro-Electron Spin Resonance (ESR) Spectrometry,” which is anon-provisional application claiming the benefit of priority ofearlier-filed U.S. Provisional Application No. 60/736,264, filed Nov.14, 2005, entitled, “In-situ measurement of automotive fluidsdegradation by micro ESR spectrometry.”

FIELD OF INVENTION

The invention relates to the field of electron spin resonance (ESR)spectrometry, and more particularly to the use of such technology formeasuring and diagnosing the real-time degradation and changes ofautomotive fluids such as engine oil and the like, in situ and duringoperating engine conditions and environments.

BACKGROUND OF INVENTION

The maintenance and monitoring of fluids in vehicles, engines, pumps,weapons and machinery (all hereinafter, for convenience, genericallyreferred to as “vehicles”) is vital to ensuring reliable operation.While there is no single sensor available that can monitor all fluidssimultaneously, due to the wide variation in composition and fluidfailure mechanisms, a suite of networked, miniaturized onboard vehiclefluid sensors can be envisioned for continuous, in-situ monitoring offluid degradation. In the case of brake fluid and hydraulic fluid, themain mechanism for fluid degradation is humidity absorption, excessparticulates (metal and sand), and solvent contamination. In-linehydraulic fluid humidity sensors are commercially available from severalsources. In the case of engine coolant, increased acidity leads tocorrosion in internal engine components. The pH monitoring of coolant isbeneficial, and could be implemented using commercial sensors (e.g.Durafet III pH electrode from Honeywell), which could be packaged foruse in vehicles by a third party. In the case of engine oil, there aredielectric [1,2], viscosity, conductivity [3], chromatic modulation [4],x-ray fluorescence, infrared and other sensors used to detect changes inthe observable fluid properties [5]. Several sensor systems areavailable which examine changes in dielectric permittivity and viscosityof oil [22], as are vehicle-specific software systems that predict oilfailure based on past driving conditions (deployed by General Motors)[6]. There are to date, however, no commercially-available sensors thatprovide a rigorous, real-time detection of the most fundamental chemicalmechanism of engine lubricating oil failure, —the formation of freeradicals by the breakdown of long hydrocarbon molecular chains in oil.Only the overall results stemming from these free radical-inducedchanges have heretofore been monitored in-situ, but not the directdetection of the free radicals themselves.

Onboard monitoring of lubricant engine oil degradation provides areduction in engine wear and reduced maintenance costs for the end-user[6]. The net economic benefit of this optimized maintenance schedule canbe very large. In the United States, over one billion gallons of motoroil are used each year; thus any reduction in oil usage can have asignificant impact. In civilian automotive applications, engine oil istypically changed every 3000-7500 miles, while coolant, brake fluid andautomatic transmission fluid are changed every 30 k-50 k miles. Theeconomic benefit to the end user of optimized engine oil management maybe greater than for other automotive fluids, both in reduced fluid costsand in reduced wear of engine components.

Using flexural mechanical structures similar to earlier U.S. Pat. Nos.5,964,242, 6,914,785 and 7,025,324, the present invention proposes tooptimize a miniature electron spin resonance (ESR) sensor for thedetection particularly, though not exclusively, of molecular peroxyradicals in engine oil and related or other fluids. The breakdown ofengine oil is indicated by a sharp increase in the concentration ofdamaging peroxy radicals (RO₂·) among others in the oil. Peroxy radicalsare readily identified by electron spin resonance (ESR) spectroscopy andthus give a clear and direct indication of the engine oil condition.

Numerous systems have before been developed by auto manufacturers andothers for improved automotive fluids management. Researchers haveprototyped the use of viscosity sensors, dielectric sensors, chromaticsensors (sensing color changes), oil pH sensors, miniature fouriertransform infrared spectrometers (FTIR) and x-ray fluorescence sensors,sensors of magnetic particles are of iron-derived and transition metalparticles and combinations thereof. General Motors employs a computermodel, which uses the car driving history, environmental conditions(temperature, humidity) and maintenance history to predict when the oilmust be changed, without specialized sensors, although detailed datafrom millions of miles of road tests was required to create thiscomputer model [6]. The present invention, however, differs from theseapproaches in a fundamental way: directly in situ sensing the molecularchanges that occur in oil as a result of breakdown of the lubricant.

OBJECTS OF INVENTION

A principal object of the present invention, accordingly, is to providea new and improved method of and apparatus for monitoring lubricant andother oil degradation and the like that, unlike prior fluid monitoringand maintenance techniques, including those above discussed, usesimproved electron-spin resonance (ESR) spectroscopy sensors andtechniques to directly sense the molecular changes that occur in suchfluids as a result of molecular breakdown therein.

A further object is to provide such novel sensors of small and miniaturesize, low cost and low power consumption and adapted for integration inautomotive engine systems and the like.

A further object is to provide a novel miniaturized ESR sensor andspectrometer of more general use and with other fluids as well.

Still another object is the providing of such a novel sensor that isadapted for implementation in onboard vehicle fluid diagnostic sensorsuites also embracing one or more of pH, dielectric, temperature andhumidity sensors and including microwave sensors of moisture content ofthe oil or other fluid and also the presence of ferromagnetic particlesand the like.

Other and further objects will be later detailed and are also delineatedin the appended claims.

SUMMARY OF INVENTION

In summary, and in one of its broader methodology aspects, the inventionembraces a method of using electron spin resonance spectrometry formeasuring the degradation of vehicle fluids, that comprises,

passing a sample of such fluid through a resonating variable RFfrequency microwave cavity resonator during the application therethroughof a constant magnetic field;

rapidly modulating the magnetic field correspondingly to vary theresonant magnetic susceptibility in such fluid sample;

modulating the RF frequency of the cavity resonator in accordance withsuch magnetic susceptibility variation; and

measuring such RF frequency modulation or amplitude modulation thereofto derive an electron spin resonance signal that directly senses themolecular changes in the fluid sample resulting from fluid degradationduring operation of the vehicle.

In its novel apparatus context, the invention provides an electron spinresonance sensor particularly adapted for use as a spectrometer having,in combination, a broadly frequency-swept high Q tunable microwavecavity resonator, provided with a fluid inlet and an outlet in its wallsfor internally passing a fluid sample through the resonator during theresonating of the cavity resonator by microwave energy in order toeffect absorption or dispersion of the microwave energy in the sample,and wherein the cavity resonator is disposed in an external uniformpermanent magnetic field of sufficient intensity to cause magneticresonance in the sample within the range of frequency sweeping.

In a still further preferred apparatus embodiment, the cavity resonatoris of re-entrant toroidal configuration sandwiched between opposingpermanent magnet structures and with a piezoelectric means extendingalong the top of the toroid, with a capacitance gap being formedexternally of the cavity between a surface of the re-entrantconfiguration and a proximate surface. The resonator and magneticfield-producing structure is of miniaturized stacked construction andadapted to be mounted onboard, in situ with operating machinery, such ason board a vehicle or other machinery monitoring the degradation oflubricant oil and other fluids.

Preferred and best mode embodiments and designs are hereinafterpresented in detail.

DRAWINGS

The invention will now be described with reference to the accompanyingdrawings in which FIGS. 1 and 2 are exploded isometric views of internalcomponents of a miniaturized micro-ESR continuous flow-through oil orsimilar fluid sensor of molecular changes that occur as a result offluid breakdown in usage, and constructed in accordance with a preferredembodiment of the invention;

FIG. 3 is a cross-sectional view on a somewhat larger scale of thesensor portion of FIGS. 1 and 2;

FIG. 4 is a block diagram showing a complete oil degradation sensingpackage and system;

FIG. 5 is an explanatory diagram of electron energy transitionsstimulated in a sample of the oil under incident microwave energy and inan applied magnetic field, showing the Zeeman splitting effect undersuch magnetic field;

FIGS. 6 and 7 are block diagrams illustrating preferred details ofelectrical sensing designs of the mini-ESR spectrometer system of theinvention;

FIG. 8 is an isometric view partly cut-away of a preferred cavityresonator construction for the sensor;

FIG. 9 is a graph of an experimentally obtained passband response forbroad tuning of a prototype sensor over a 1.3-2.6 GHz microwave band;

FIG. 10 is a diagram of a preferred design of a permanent magnetassembly with uniform magnetic field in accordance with the invention;

FIG. 11 presents a sample of the signal output from a zero-fieldminiature ESR spectrometer of the type shown in FIG. 7; and

FIG. 12 presents signal-processed experimentally obtained intensity vs.g-factor graphs of an X-band (9.80 GHz) ESR spectrum obtained for newand used motor oil.

DESCRIPTION OF PREFERRED EMBODIMENT(S) OF INVENTION

Before proceeding to describe preferred sensor structures and circuitsfor practicing the invention, it is believed to be helpful briefly toreview the principles and prior implementation of electron spinresonance (ESR) spectrometry.

Electron Spin Resonance (ESR) Spectrometry

An electron spin resonance (ESR) spectrometer detects the concentrationand composition of free radicals present in, for example, an oil sample.The sample is loaded into a high frequency microwave resonant cavity ina magnetic field H. Free radicals irradiated with microwave radiationwill undergo transitions at a characteristic frequency governed by thefollowing equation (1), and as shown conceptually in FIG. 5 which showsthe before-mentioned Zeeman—effect splitting under the applied magneticfield:hv=gBH  (1)

In this equation, h is Planck's constant, B is the Bohr Magneton, v isthe resonant frequency, H is the applied magnetic field, and g is acharacteristic of the radical (the “g-factor,” a number, often close to2.0000). The absorption of incident microwave energy has acharacteristic resonant peak, as shown in FIG. 9. The frequency (ormagnetic field) at resonance is a function of the g-factor, and theheight of the resonant peak is determined by the concentration of theradical in the sample.

Historically (since 1945), ESR spectrometers have used largeelectromagnets to generate a variable magnetic field, and have employedfixed-frequency cavities. This is a similar arrangement to that found ina nuclear magnetic resonance (NMR) spectrometer. In terms ofportability, this design has been a significant hindrance since thetunable electromagnet magnet assembly weighs upwards of 200 kg andrequires water cooling and several kW of power for operation. Themicro-ESR sensor of the present invention has circumvented this problemby using a small, strong permanent magnet assembly such as of rare-earthelements or other permanent constant field magnets that generate afixed, uniform magnetic field (700 Gauss, for example), together with abroadly tunable (not fixed-frequency) microwave cavity (tunable from 1.3to one hundred percent higher 2.6 GHz, for example, as in FIG. 9). Thisbroadly tunable, high-Q cavity resonator is an enabling technology inthe micro-ESR sensor design and operation of the invention.

Structural and Electrical Design

Turning, now, to the sensor construction of the present invention asillustrated in the cross-sectional view of FIG. 3 and the isometricexploded views of FIGS. 1 and 2, a sandwich type stacked assemblypackage P is provided wherein preferably a toroidal-shaped integralconductive-walled chamber 106 bounds internally a microwave resonanttoroidal annular cavity chamber 100 excited in conventional well-knownmanner at RF excitation feed 112 as schematically represented at RF. Thecavity, as before mentioned, is covered along the top wall 1061 by anattached planar electric-to-mechanical transducer such as apiezoelectric disc element or film 102, and rests along the bottom on anintegral conductive planar base plate 103, similarly to structurestaught in our copending U.S. patent application Ser. No. 11/392,980,filed Mar. 28, 2006 for a Variable Electrical Circuit Component and inour later-referenced IEEE article. Under the present invention, however,a sample of the fluid-to-be-monitored, such as the before-mentionedexemplary engine oil or lubricant, is carried into one side of thecavity (shown as the top left A in FIGS. 1 and 2), passed through thecavity and flowing out the other side B through a dielectric fluidtubing loop 107 inserted within the cavity. The bottom of the re-entrantcenter column R of the toroidal chamber 106 is closed by a wall 105, astaught in the before-referenced US patents, (sometimes referred to bythe trade name Aesop “Nanogate” technology) closely positioned to left-and right-hand electrical coupling structures 108-112 and 109-113 onopposite sides of the center portion 101 of the base 103, as moreparticularly shown in FIG. 3, wherein the slot 113 serves as the RFoutput probe feed. The elements 105 and 101 define a gap G serving as acapacitor in the overall resonating circuit, with the toroidal cavity100 serving as the inductance of the resonating circuit.

At the top of the cylindrical sandwich sensor unit P, above thepiezoelectric element 102, a preferably circular permanent magnet 99 isexternally mounted (FIGS. 1 and 2), and at the bottom, below the plate103, it acts with an opposing similar magnet 98 to set up theearlier-mentioned uniform, fixed or constant magnetic field H passingthrough the annular cavity 100 in the vertical-arrowed direction 110.Magnetic field modulating Helmholtz-type coils 88 and 89 may also beprovided for rapid modulation of the magnetic field (for example at afrequency of about 10 KHz) to provide for synchronous detection ofmagnetic resonance as later explained. The magnetic coil modulationfrequency may typically be between 1 KHz and 100 KHz. Below 1 KHz,vibrations tend to reduce the spectrometer sensitivity. At more than 100KHz, the synchronous detector (lock-in amplifier) later discussed inconnection with the system of FIG. 6 is difficult to implement usinglinear circuits. The fluid sample itself, moreover, has a finiteresponse time which would limit the maximum magnetic field modulationfrequency. The return fixed magnetic field flux path is along the outercasing of the whole instrument (schematically represented at C), alongthe arrows 111 in FIG. 3, with the casing C preferably constructed of amaterial with high magnetic permeability such as mild steel or the like.

When the piezoelectric disc or film 102 at the top wall of the toroidalresonant chamber deforms—contracts or expands—in response to appliedvoltage at 3, it flexes the top wall 1061 as an elastically deformablediaphragm and thus flexes the toroidal cavity shape, causing variationof the gap spacing G of the before-described external capacitancedefined between the center bottom wall portion 105 and the opposing wall101 of the base 103, and thus providing variable capacitance tuning ofthe resonating circuit. The integral constraining cavity side walls actas an elastic fulcrum mechanically supporting the outer edge of thecavity flexible top wall 106 ¹, such that the vertical force produced inthe central region R by the expansion or contraction of thepiezoelectric disc 102 creates amplified enhanced tuning displacementsin the gap G. The structure of the resonant cavity 100-106 thus itselfprovides an elastically deformable fulcrum which is deformed by thepiezoelectric actuator 102, and serves to amplify the motion of thecavity top surface 106 ¹ in a manner similar to that described in saidpatents.

The invention therefore uses a permanent constantmagnetic-field-producing assembly stacked with and passing a magneticfield through the cavity resonator 100-106 while its conductive internalcavity space is electrically resonating at an RF microwave frequencywith its integral but external capacitive gap G. The return magneticflux path, as before stated, is the outer instrument casing C. This isin direct contrast to prior-art ESR detectors which, as earliermentioned, have employed varying (not fixed) magnetic fields thatrequire the before-mentioned large-size magnetic-field generating coils,and are used with a fixed (not variable) cavity resonant frequency,necessitating the very large-size construction of the prior ESR systems.Through the use of the permanent magnets of fixed field and externalvariable capacitor with a very small gap, miniaturization of the stackedstructure of the present invention is enabled, and this, in turn, makespossible an on-board in-situ mounting and use at the engine.

The installation of the sensor package stack P of the invention in anexemplary oil degradation sensor system is shown in FIG. 4, wherein asmall pump 52 drives oil from the engine E at 50 (“oil in”) into theinlet A and through the hollow tubing loop 107 inside the resonantcavity 100, exiting at B and 49 (“oil out”) such that a sample of theoil (or other fluids) can be continuously introduced into and withdrawnfrom the sensor in a controlled and continuous pass-through manner. Alower cost gravity-fed design or an arrangement where the sensor isfitted to pressurized oilways in the engine may also be used. An in-linefilter 51 may prevent clogging of the tubing (2 mm ID fine tubing, forexample) in the resonator assembly. By making a sensor with largerinternal channels, a filter may indeed be rendered unnecessary. Theoverall miniaturized sensor package of FIG. 4 may be about two incheswide and one inch high.

Breakdown of Engine Oil

Oxidation of petroleum hydrocarbons proceeds by a radical chainmechanism via alkyl and peroxy radicals after an induction periodwherein the antioxidants in the oil are consumed. The chain is initiatedas follows:RH→R·(free radical)+H·RH+O₂→R·(free radical)+HO₂·

The chain then propagates as:R·+O₂→RO₂·(peroxy radical)RO₂·+R′H→ROOH (hydroperoxide)+R′·Oxidation reactions can result in premature degradation of the basefluid (i.e. the formation of acids, gums, lacquers, varnishes andsludges) at prolonged high temperatures. Mineral oil fractions alreadycontain natural inhibitors in the form of sulphur and nitrogencompounds, aromatics or partially hydrogenated aromatics, phenolicoxidation products, etc., which delay oxidation and impart good ageingproperties. However, when the mineral oil is subjected to a high degreeof refining, these materials can be lost along with those havingless-desirable characteristics [18].

Exemplary ESR Spectrometer Systems

In FIG. 6, a 90° hybrid 217, the electrical cavity resonator packageP-100, a phase adjuster 210, and a phase detector 209 are shown arrangedas a frequency discriminator circuit. Frequency modulation of theresonator cavity 100 causes phase modulation of the RF signal coupledthrough it. The phase modulation of the RF carrier is demodulated by thephase detector 209 which may be implemented as a mixer operating withthe RF and the inputs from a local oscillator 208, in quadrature. Theoscillator phase is adjusted at 210 for such quadrature phase detection.

The spectrometer operates via a slow frequency sweep (so-labeled at 215)of the voltage-controlled local oscillator (VCO) 208. A low bandwidthintegrator servo loop is used with a controller 221 to adjust the cavityelectrical resonator frequency to the frequency of thevoltage-controlled oscillator 208. The bandwidth of the servo loop may,however, in some instances, be too slow in this type of embodiment tocompensate phase modulation by the magnetic modulation field introducedby the Helmholtz coils at 88 and 89. The sensor piezo driver is thuswithin the servo loop that adjusts the frequency of the resonant cavity100 to the frequency of the swept oscillator 208. The bandwidth of thisloop is on the order of Hz, such that it does not respond to changes inmagnetic resonance caused by the modulation field.

Magnetic resonance causes a change in the magnetic susceptibility of theoil sample passed through the cavity resonator at a frequency dependingon the Zeeman field at the fluid sample (FIG. 5). The modulation of themagnetic field applied by the modulation driver and Helmholtz coils(˜0.1-10 Gauss amplitude) varies the Zeeman field at the sample andtherefore the frequency of magnetic resonance. At a given measurementfrequency, the modulation of the magnetic susceptibility of the fluidsample modulates the RF frequency of the cavity resonator. The frequencymodulation of the cavity resonator is measured by the above-describedfrequency discriminator circuit, including the frequency counter 213inputting a data computer 214, and synchronously detected at basebandusing the servo loop and lock-in amplifier 220 also feeding the datacomputer 214, all schematically represented in FIG. 4 as the “sensorelectronics 120”. Such measurement provides an electron spin resonancesignal that directly indicates the molecular changes in the fluidsamples resulting from fluid degradation during operation of thevehicle.

In a second implementation shown in FIG. 7, frequency modulation of theRF carrier is used spectrometrically to detect resonance in the magneticsusceptibility of the fluid sample. In this design, the integrator-servobandwidth is larger than the modulation frequency, so that theelectrical cavity resonator frequency tracks the modulation of thefrequency swept voltage-controlled oscillator 208. As before stated,this circuit uses the integration (206) servo feedback loop F and alsovaractor tuning 207 as labeled. Magnetic resonance is detected viachanges in insertion loss of the cavity resonator, which causesamplitude modulation of the RF carrier. The RF signal amplitudemodulation is demodulated at 205 and synchronously detected using thebefore-mentioned phase lock-in amplifier 220.

The signal from the phase detector is used to lock the cavity resonatorfrequency to the frequency modulation of the voltage-controlledoscillator 208. The RF amplitude detector 205 measures changes in theinsertion loss of the cavity resonator 100 which are caused by theparamagnetic resonance absorption of the oil sample passed through theloop 107 in the cavity resonator 100. The system requires carefulmatching and power leveling of the VCO signal to avoid spurious AMbackground. Additional components, such as directional couplers 218,221, attenuators 219, 211 buffer amplifiers and isolators, may be usedto improve the design, if desired.

The design of the frequency-swept ESR spectrometer of the invention isthus fundamentally different from that of conventional ESR spectrometerssince the quantity measured is both phase dispersion and amplitudevariation. The spectrometer is a novel variation in which the electronspin resonance signal is detected via the amplitude modulation of thetransmission coupled RF carrier, since the automatic frequency controlloop is configured to follow the frequency modulation of the VCO215-208.

The preferred RF frequency tuning mechanism for the invention is shownbased on the before-mentioned “Nanogate” mechanical diaphragm flexure ofthe cavity top wall 106 ¹ in response to the action of the piezoelectricelement 102 and is used to precisely tune the spacing G between thecapacitor electrodes 105-101 of the sensor, FIG. 3.

Recent experimental work has demonstrated that a simple, tunable LCresonator structure of this type, FIG. 8, which uses a piezoelectricdrive 102 and has miniature dimensions approximately 13 mm by 13 mm by 3mm height, enables very broad tuning and with high-Q over a 1.3-2.6 GHzband, and a quality factor of 380. This is the type of structure we havedescribed in our article “Octave-Tunable Miniature RF Resonators”appearing in the IEEE Microwave and Wireless Components Letters, Vol.15, No. 11, pages 793 on, November 2005. As shown in FIG. 9, the centerfrequency of the resonator can be continuously varied to any frequencybetween, in this case, 1.3 GHz and 2.6 GHz, by adjusting the voltageapplied at 3 to the piezoelectric actuator 102.

Sensitivity Considerations

Considering, now, the achievable sensitivity of sensors constructed inaccordance with the present invention, an experimental comparison of thesensitivity of the mini-ESR spectrometer of the invention to the typicalsensitivity of a classic relatively fixed-frequency 10 GHz X-band EPRspectrometer design, demonstrated that the sensitivity is equal orbetter for limited sample sizes. The key parameters in comparing thesensitivity of different spectrometer designs are the resonator qualityfactor Q, the fill factor η, and the dielectric loss of the sample. Theresult of the analysis is that while X-band cavities have very high-Qs(˜5000), the sensitivity in most practical cases is on par with smallerre-entrant high-Q cavity type resonators because the fill factor is muchlarger for re-entrant cavities. This result is well documented in theliterature on ESR (e.g. [8, 9, 10]) and explains the increasing use ofloop-gap and re-entrant type resonators.

Re-entrant cavity and loop-gap resonators have Qs of the order of 500,but much larger fill factors than X-band cavity resonators. For limitedsample sizes or aqueous samples, the sensitivity of loop-gap resonatorsis equivalent or better than traditional X-band cavity resonators [8, 9,10]. The exception is for large samples of low loss dielectric. TheOctave+resonators are ideal for use in the ESR detection of theinvention due to the combination of high-Q (˜500) and a small volume. Inaddition, the isolation of the magnetic field region within the cavity100 from the external capacitive gap G reduces the sensitivity of thecavity to dielectric loss, which is important for fluid samplescontaining water. X-band cavities have dramatically reduced Q for largesamples containing water. Even for small samples, placement at thecenter of the cavity (node in the electric field for a TE₁₀₂-excitedcavity) is essential to minimize the degradation of the cavity Q due tosuch dielectric loss.

The fill factor for a TE₁₀₂ mode X-band cavity is:

$\begin{matrix}{\eta = \frac{2V_{s}}{V_{c}}} & (2)\end{matrix}$where V_(s) is the sample volume and V_(c) is the cavity volume, in therange of 10 cc [11]. A typical sample volume is in the range of 0.1 cc,which means that the fill factor for an X-band cavity is in the range of2% in practice. In contrast, loop-gap and re-entrant cavity resonatorsare designed for much higher fill factors at small sample volumes[12,13,14,15,7]. The Octave+tunable resonators are ideal to achieve acombination of large fill factor and high-Q, quality factor, which maybe compared to X-band cavities and conventional loop-gap resonators asin Table 1 below:

TABLE 1 X-Band Loop-Gap Octave⁺ Tunable Resonator Quality Factor Q 5 000500 500 Fill Factor η 2% 20% 30% FOM Sensitivity   100 100 150

As mentioned above, the ESR sensitivity is proportional to the productof the fill factor and Q, and the performance of different spectrometerscan be compared using the product of these two parameters, as also shownin Table 1. Loop-gap resonators have sensitivity on par with X-bandcavities for typical samples, and for the Octave+tunable resonator, a50% improvement in sensitivity is expected because of the carefuloptimization of the quality factor and resonator volume. Again, thesensitivity comparison of cavity and loop-gap resonators is well knownin ESR research [8,9,10].

In practice, an RF microwave cavity design where the large electric andlarge magnetic field regions are segregated, as in the construction ofFIGS. 1, 2 and 3 of the invention, is important to realize a high-Qresonator. Dielectric absorption of the RF electric field via watermolecules in the sample decreases the resonator Q. Only the RF magneticfield, however, excites magnetic resonance. In reference to thebefore-described FIG. 8, the “inductive” region of the re-entrant cavity100 is the internal toroidal chamber space or region containing thesample. In this region, the magnetic field is large and the electricfield is small. The “capacitive” region, as before explained, where theelectric field is most intense, is the capacitive gap G, external to thecavity. Thus, in the re-entrant cavity design of the invention, theeffect of dielectric absorption in the fluid sample in the cavity-Q isminimized by restricting or confining or isolating the placing of thesample to or in the inductive internal cavity region 100 only. Theoverall fill factor may still be large, however, since the resonantenergy is stored equally in the magnetic and electric fields

The absolute sensitivity of a spectrometer in terms of the measurablenumber of spins can be quantified by calculating the amplitude and phasemodulation introduced by paramagnetic resonance. The effectiveinductance L of the loop-gap resonator or other resonator containing theparamagnetic sample is modified by the change in magnetic susceptibilityas given by the expression:

$\begin{matrix}{L = {L_{0}\left\lbrack {1 + {4{{\pi\eta}\left( {\chi^{\prime} - {i\;\chi^{''}}} \right)}}} \right\rbrack}} & (3)\end{matrix}$where χ′ and χ″ are the real an imaginary parts of the magneticsusceptibility (i.e. dispersion and absorption.) The fill factor η isratio of RF magnetic field stored in the fluid sample and the magneticfield in the entire cavity:

$\begin{matrix}{\eta = \frac{\int_{sample}{H_{1}^{2}{\mathbb{d}v}}}{\int_{cavity}{H_{1}^{2}{\mathbb{d}v}}}} & \left( {3A} \right)\end{matrix}$

The fluid sample contained in the resonant circuit results in aperturbation to the frequency and loss of the cavity. The phaseperturbation introduced by paramagnetic resonance by a signal coupled intransmission is:

$\begin{matrix}{{\partial\theta} = {{- 4}{\pi\eta}\; Q_{L}\chi^{\prime}}} & (4)\end{matrix}$and the change in the resonator quality factor due to magnetic resonanceis:

$\begin{matrix}{\frac{\Delta\; Q}{Q_{L}} = {{{- \frac{Q_{L}}{Q_{L} + Q_{m}}} \cong {- \frac{Q_{L}}{Q_{M}}}} = {4{\pi\eta}\; Q_{L}\chi^{''}}}} & (5)\end{matrix}$where Q_(L) is the loaded-Q of the electrical resonator. Theparamagnetic resonance frequency is modulated by using the Helmholtzcoils to modulate the Zeeman field. The phase and amplitudeperturbations described by equations (4) and (5) can be used tocalculate the power of the phase and amplitude modulation introduced onthe RF microwave carrier by the magnetic resonance absorption. Thissideband power is equated to the noise floor of the receiver to find theminimum detectable number of spins:

$\begin{matrix}{N_{\min} = {\frac{V_{s}}{4{\pi\eta}\; Q_{L}}\frac{kT}{\left( {g_{S}\mu_{B}} \right)^{2}}\frac{\Delta\; v}{v_{0}}\sqrt{\frac{kT}{P_{S}}}}} & (6)\end{matrix}$where P_(s) is the carrier power, Δυis the ESR resonance width, and υ₀is the ESR resonant frequency. For example, using typical parameters,assume a 0.1 cc sample volume, Q_(L)=500, fill factor ¼, and a 5 Gaussline at 1000 Gauss field:

$\begin{matrix}\begin{matrix}{N_{\min} = {\frac{0.1{cc}}{4{\pi \cdot 0.25 \cdot 500}}\frac{1}{{8.29e} - 27}\frac{5}{1000}\sqrt{\frac{{4.14e} - {21{J \cdot 1}\mspace{14mu}{Hz}}}{10^{- 3}W}}}} \\{= {3.84 \times {10^{19} \cdot 2} \times 10^{- 9}}} \\{= {7.8 \times 10^{10}}}\end{matrix} & (7)\end{matrix}$Equation (7) is the “rule of thumb” sensitivity of 10¹¹ spins/mT for ESRspectrometers, i.e., for a 10 Gauss ESR resonance, the minimummeasurable number of spins is 10¹¹ in a 1 Hz bandwidth. For somespectrometer designs, the phase and amplitude noise of the microwavesource and vibrations, may be limiting factors for the sensitivity.However, with refinements to the design, sensitivity limited by thethermal noise floor of the receive chain can be realized (c.f. [7] forcomplete discussion)

Permanent Magnet Design

As before explained, the feature of being able to use permanent magnetsfor miniaturization and other purposes requires, however, careful designof the shape and positioning of the permanent magnets in the sensorassembly P in order to maximize the sensitivity of the sensor. Oneapproach is to grind the faces of the magnets with a curved shape as in[16] and FIG. 10, which extends the region of field homogeneitysubstantially. For the present invention, a low-frequency finite-elementmodeling program called Maxwell™ may be used numerically to optimize theshape of the magnet face, as shown conceptually in FIG. 10. Rare earthNdFeB or other permanent magnets 98,99 may be custom-ground to theoptimized shape. In high-volume production, the field magnets would besintered directly with the optimal curved face, thus the final cost inproduction would be little more than for flat-faced magnets.

Experimental Results

In order to provide a baseline reference for a pure oil “signature” toserve as a basis for comparison with deteriorating oils in the field,ESR spectrum performance at a zero reference magnetic field must beknown. A sample of the signal output from the zero-field miniature ESRspectrometer of the present invention is shown in FIG. 11. This is thespectrum from a Vanadium-doped Magnesium Oxide crystal placed in thecavity resonator 100.

Experimental investigation results of the radical concentrations in usedengine oil are shown in FIG. 12. Engine oil from a brand-new 2005Chrysler Sebring at 220 ml. and 1709 ml, and an additional sample ofused oil (used for 6000 miles) from a 1990 Honda Accord were obtainedand tested in a Bruker Biospin ESR. The experiment used X-band (9.80GHz) ESR spectra of new and used motor oil. All samples showed thecharacteristic ESR signal at g=2.0055 corresponding to the peroxyradical responsible for oil breakdown [19]. An additional signalcorresponding to carbon radicals was also observed, which shifts thezero-crossing of the ESR signal, as shown. Signal-processing techniqueswere employed to separate the signal due to carbon radicals from thesignal due to the peroxy radical. In this case, each signal can beapproximated as the first derivative of a Lorentzian curve, and giventhe overall peak width, peak height, zero-crossing point, and theg-values of each radical, with a simple iterative algorithm determiningthe best fit for each radical species.

In the course of this specification, bracketed numbers have been used torefer to published reference materials of relevance. These are nowsummarized in the following tabulation:

-   [1] A. A. Carey and A. J. Hayzen, CSI, “The Dielectric Constant and    Oil Analysis”. Practicing Oil Analysis Magazine, September 2001.-   [2] R. D. Lee, H. J. Kim, Y. P. Semenov, “Capacitive Sensor for in    situ Measurement of Deterioration of Car Engine Oil,” The Korean    Sensors Society, Vol. 10, No. 4, pp. 266-272, 2001.-   [3] Basu, A. et al. “Smart Sensing of Oil Degradation and Oil Level    Measurements in Gasoline Engines,” SAE Technical Paper 2000-01-1366,    SAE World Congress, Detroit Mich. Mar. 6-9, 2000.-   [4] I. I. Khandaker, “A fibre-optic oil condition monitor based on    chromatic modulation,” Meas. Sci. Technol. 4 (1998) 608-613.-   [5] J. D. Turner and L. Austin, “Electrical techniques for    monitoring the condition of lubricating oil,” Meas. Sci Technol.,    14 (2003) 1794-1800.-   [6] B. W. Wilson et. al. “Development of a modular in-situ oil    analysis prognostic system,” Int. Soc. Logist. Symp., pp. 1999.-   [7] C. White, “A Solid-State Atomic Frequency Standard” Ph.D Thesis,    California Institute of Technology, 2005.-   [8] A. J. Hoff ed., Advanced EPR: applications in biology and    biochemistry, Elsevier, Amsterdam, 1989, p. 282.-   [9] S. Pfenninger et al., “Bridged Loop-gap Resonator: A resonant    structure for pulsed ESR transparent to high-frequency radiation,”    Rev. Sci. Instrum., 59, 752, 1988.-   [10] W. Piasecki, W. Froncisz, and James S. Hyde, “Bimodal Loop-gap    Resonator,” Rev. Sci. Instrum., 67, 1896, 1996.-   [11] Charles P. Poole, Electron Spin Resonance: A Comprehensive    Treatise on Experimental Techniques, 2nd ed., Wiley 1983, c.f. p    395.-   [12] S. Pfenninger, W. Froncisz, J. Forrer, J. Luglia, and James S.    Hyde, “General Method for Adjusting the Quality Factor of EPR    Resonators,” Rev. Sci. Instrum., 66, 4857, 1995.-   [13] T. Christides, W. Froncisz, T. Oles, James S. Hyde, “Probehead    with Interchangeable Loop-gap Resonators and RF Coils for    Multifrequency EPR/ENDOR,” Rev. Sci. Instrum., 65, 63, 1994.-   [14] W. Froncisz and James S. Hyde, “The Loop-gap Resonator: A New    Microwave Lumped Circuit ESR Sample Structure,” J. Magnetic    Resonance, 47, 515, 1982.-   [15] W. N. Hardy and L. A. Whitehead, “Split-ring Resonator for use    in Magnetic Resonance from 200-2000 MHz,” Rev. Sci. Instrum., 52,    213, 1981.-   [16] Rupp, L. W. et al, “Miniature magnet for electron spin    resonance experiments,” Am. J. Phys. V. 44, no. 7, July 1976, pp.    655-657.-   [17] A. H. Price and G. H. Wegdam, “Dielectric spectroscopy at    microwave frequencies,” J. Phys. E, vol. 10, pp. 478-481, 1977.-   [18] “Emission Scenario Document on Lubricants and Lubricant    Additives,” OECD, www.oecd.org/ehs/, December 2004.-   [19] Ikeya, M. “Car Mileage Determination with ESR Signal of Engine    Oil: A Case of Organic ESR Dating,” ESR Dating and Dosimetry,    IONICS, Tokyo, 1985, pp. 453-457.-   [20] J. W. Waters et. al., “Remote Sensing of Atmospheric Water    Vapor and Liquid Water with the Nimbus 5 Microwave Spectrometer,” J.    Appl. Meteo., vol. 15 pp. 1204-1208, 1976.-   [21] Aesop Inc. Datasheet for Octave+2.5-5.0 Ghz miniature tunable    resonator, www.activespectrum.com, 2005.-   [22] D. R. Sparks et. al. “Application of MEMS Technology in    Automotive Sensors and Actuators,” Int. Symp. Micromech. And Human    Sci., pp. 9-16, 1998.-   [23] U.S. Pat. No. 4,803,624, issued Feb. 7, 1989, to Pilbrow et    al., discloses a portable electron spin resonance spectrometer.-   [24] Hiroshi Hirata, Toshifumi Kuyama, Mitshuhiro Ono, and Yuhei    Shimoyama, “Detection of electron paramagnetic resonance absorption    using frequency modulation,” Journal of Magnetic Resonance,    164 (2003) 233-241.

While the invention has been described with particular reference to theimportant application for in situ on-board monitoring of the state ofdegradation of vehicle engine fluids, the novel ESR sensor constructionof the invention may also be usefully employed with other fluids andmaterials and in a myriad of other applications including in otherfields; and further modifications will therefore occur to those skilledin this art, such being considered to fall within the spirit and scopeof the invention as defined in the appended claims.

What is claimed is:
 1. In the operation of an electron spin resonancesensor, a method of monitoring the degradation of lubricating oil inoperating conditions, that comprises, detecting, in a flow througharrangement, the development and concentration of free radicals producedduring oxidation by directly detecting the paramagnetic resonance ofsuch radicals as the lubricating oil flows, under pressure, through theelectron spin resonance sensor.
 2. The method as claimed in claim 1,wherein signal processing is employed to separate signals derived fromthe detection of the radicals produced during oxidation from signals dueto other radicals such as carbon.
 3. A method for predicting a remaininguseful life of a lubricating oil, comprising acts of: passing thelubricating oil, in a flow through arrangement, through an electron spinresonance sensor; detecting with the electron spin resonance sensor, inthe lubricating oil, a concentration of radicals produced duringoxidation; and determining with the electron spin resonance sensor,based on the concentration of radicals, a remaining useful life of thelubricating oil.